Walk up and politely change the subject. If that doesn't work, politely ask them to stop. If that doesn't work, give up and tell someone about it later. If your relationship at this point is such that it will affect your ability to communicate during flight, call the chief and boot them or yourself from the trip.
This is a scenario straight from the new hire interview. Use some tact and don't be an a hole. Captains this is your job at all times in uniform. FO's without CA around. On or off the plane. Including the hotel lobby.
Bottom line is don't be jerk and be safe. Treat everyone with respect, even when they don't respect themselves.
If you need to do some extra work on your day off, God forbid, just do it. It's part of the profession.